Positive and negative costimulatory signals play critical roles in the modulation of B and T cell activity, and the molecules that mediate these signals have proven to be effective targets for immunomodulatory agents. Positive costimulation, in addition to T cell receptor (TCR) engagement, is required for optimal activation of naive T cells, whereas negative costimulation is believed to be required for the acquisition of immunologic tolerance to self, as well as the termination of effector T cell functions. Upon interaction with B7.1 or B7.2 on the surface of antigen-presenting cells (APC), CD28, the prototypic T cell costimulatory molecule, emits signals that promote T cell proliferation and differentiation in response to TCR engagement, while the CD28 homologue cytotoxic T lymphocyte antigen-4 (CTLA-4) mediates inhibition of T cell proliferation and effector functions (Chambers et al., Ann. Rev. Immunol., 19:565-594, 2001; Egen et al., Nature Immunol., 3:611-618, 2002).
Several new molecules with homology to the B7 family have been discovered (Abbas et al., Nat. Med., 5:1345-6, 1999; Coyle et al., Nat. Immunol., 2: 203-9, 2001; Carreno et al., Annu. Rev. Immunol., 20: 29-53, 2002; Liang et al., Curr. Opin. Immunol., 14: 384-90, 2002), and their role in T cell activation is just beginning to be elucidated. These new costimulatory ligands include B7h, PD-L1, PD-L2, and B7-H3.
B7h (Swallow et al., Immunity, 11: 423-32, 1999), also known as B7RP -1 (Yoshinaga et al., Nature, 402: 827-32, 1999), GL50 (Ling, et al., J. Immunol., 164:1653-7, 2000), B7H2 (Wang et al., Blood, 96: 2808-13, 2000), and LICOS (Brodie et al., Curr. Biol., 10: 333-6, 2000), binds to an inducible costimulator (ICOS) on activated T cells, and costimulates T cell proliferation and production of cytokines such as interleukin 4 (IL-4) and IL-10.
PD-L1 (Freeman et al., J. Exp. Med., 192: 1027-34, 2000), also known as B7-H1 (Dong et al., Nat. Med., 5, 1365-9, 1999), and PD-L2 (Latchman et al., Nat. Immunol., 2:261-8, 2001), also known as B7-DC (Tseng et al., J. Exp. Med., 193, 839-46, 2001) bind to programmed death 1 (PD-1) receptor on T and B cells.
Finally, B7-H3 binds an as yet currently unknown counter-receptor on activated T cells, and is reported to enhance proliferation of CD4+ T helper (Th) cells and CD8+ cytotoxic T lymphocytes (CTLs or Tcs) and selectively enhance IFN-γ expression (Chapoval et al., Nat. Immunol., 2, 269-74, 2001; Sun et al., J. Immunol., 168, 6294-7, 2002). B7-H3 has also been reported to be a negative regulator (Suh et al., Nat. Immunol., 4, 899-906, 2003; Prasad et al., J. Immunol., 173, 2500-2506, 2004). B7-H3 has also been reported to be a negative regulator (Suh et al., Nat. Immunol., 4, 899-906, 2003; Prasad et al., J. Immunol., 173, 2500-2506, 2004).
The identification of additional molecules that have T cell costimulatory activity is of keen interest due to their fundamental biological importance and the therapeutic potential of agents capable of affecting their activity. Agents capable of modulating costimulatory signals, and thereby capable of modulating the activation and/or effector functions of CD8+ CTLs and CD4+ Th cells find use in the modulation of immune responses, and are highly desirable.
In particular, many autoimmune disorders are known to involve autoreactive T cells and autoantibodies. Agents that are capable of inhibiting or eliminating autoreactive lymphocytes without compromising the immune system's ability to defend against pathogens are highly desirable.
Conversely, many cancer immunotherapies, such as adoptive immunotherapy, expand tumor-specific T cell populations and direct them to attack and kill tumor cells (Dudley et al., Science 298:850-854, 2002; Pardoll, Nature Biotech., 20:1207-1208, 2002; Egen et al., Nature Immunol., 3:611-618, 2002). Agents capable of augmenting tumor attack are highly desirable.
In addition, immune responses to many different antigens (e.g., microbial antigens or tumor antigens), while detectable, are frequently of insufficient magnitude to afford protection against a disease process mediated by agents (e.g., infectious microorganisms or tumor cells) expressing those antigens. It is often desirable to administer to the subject, in conjunction with the antigen, an adjuvant that serves to enhance the immune response to the antigen in the subject.
It is also desirable to inhibit normal immune responses to antigen under certain circumstances. For example, the suppression of normal immune responses in a patient receiving a transplant is desirable, and agents that exhibit such immunosuppressive activity are highly desirable.
Costimulatory signals, particularly positive costimulatory signals, also play a role in the modulation of B cell activity. For example, B cell activation and the survival of germinal center B cells require T cell-derived signals in addition to stimulation by antigen. CD40 ligand present on the surface of helper T cells interacts with CD40 on the surface of B cells, and mediates many such T-cell dependent effects in B cells.
The protein BTLA (B and T lymphocyte attenuator) is a member of the CD28 family of receptors that also includes CD28, CTLA-4, ICOS, and PD-1. The initial members of the family, CD28 and ICOS, were discovered by functional effects on augmenting T cell proliferation following the addition of monoclonal antibodies (Hutloff et al. (1999) Nature 397:263-266; Hansen et al. (1980) Immunogenics 10:247-260). BTLA was discovered through screening for differential expression in TH1 cells. In addition, BTLA has been described as providing negative stimulatory signals, analogous to CTLA-4. In the presence of agonist anti-BTLA mAb, anti-CD3 and anti-CD28 activated T-cells show reduced IL-2 production and proliferation (Kreig et al., J. Immunol., 175, 6420-6472, 2005). Mice lacking an intact BTLA gene show higher titers to DNP-KLH post-immunization and an increased sensitivity to EAE (Watanabe et al., Nat. Immunol., 4, 670-679, 2003). HVEM (herpes virus entry mediator) is thought to be a ligand for BTLA (Scully et al. (2005) Nat. Immunol. 6:90-98; Gonzalez et al. (2005) Proc. Nat'l. Acad. Sci. U.S.A. 102: 1116-1121).
Accordingly, agents that recognize BTLA, and methods of using such agents, are desired.